Modelling of Multiphase Flow in Pressure Swirl Atomizer

Transcription

1 International Journal of Engineering an Technology olume 6 No.5, May, 216 Moelling of Multiphase Flow in Pressure wirl Atomizer.K. Ameorme chool of Mechanical Engineering, Institute of Thermofluis an Combustion, University of Lees, UK ABTRACT The internal flow in a pressure-swirl atomizer has been stuie numerically. The atomizer is a generic esign intene for internal combustion engines an is to be operate with liqui propane C 3 H 8 as LPG is graually replacing gasoline in internal combustion engines. The main objective of the work is to moel the internal interaction of liqui propane (C 3 H 8 )-air flow in the atomizer using commercial coe TAR-CCM+. pecial emphasis is given to the flow of the liqui phase. Contour plots of axial an tangential velocities are obtaine. The volume fraction of propane C 3 H 8 an air are also presente. In aition, scalar scene of the vorticity is shown an analyse. Computational flui ynamics, CFD, simulations have been performe in a three-imensional, non-uniform gri on the atomizer an the computational omain. The liqui-air flow is moelle with the volume of flui (OF) multi-phase moel in TAR-CCM+. The simulations manage to capture the overall flow characteristics in the pressure-swirl atomizer. Key wors: olume Of Flui (OF), Propane (C 3 H 8 ), orticity, Axial an Tangential elocities 1. INTRODUCTION Atomizers are use in many engineering applications incluing spray combustion in furnaces, iesel engines, irect injection petrol engines an gas turbine engines. They are also commonly use in applying agricultural chemicals to crops, paint spraying, spray rying of wet solis, foo processing an cooling of nuclear cores. They are use to increase the specific surface area of the fuel an thereby achieve a high rate of mixing an evaporation. In most combustion systems reuction in mean rop size leas to higher volumetric heat release rate, easier ignition, a wie burning range an lower exhaust concentrations of the pollutant emissions. Atomizers are known to affect combustion stability limits, combustion efficiency, smoke an carbon monoxie generation, an unburne hyrocarbons [1, 2]. Pressure atomizers have ifferent esigns such as plain orifice, simplex, uplex, ualorifice, fan spray an spill return. Pressure swirl atomizers also calle simplex atomizers are the most versatile among them [3]. In all of them pressure swirl atomizers occupy a special position because they iffer in quality of atomization, simplicity of construction, reliability of operation, low clogging an low expeniture of energy. However, the greatest isavantages of these atomizers are that they require very high injection pressure an have low ischarge coefficient owing to the fact the air core covers the majority of the atomizer orifice [4-6]. The basic principle of flui flow through the swirl atomizer is that the liqui is introuce through tangential or helical passages into a swirl chamber from which it emerges through an exit orifice with a tangential velocity components. As a result of the vortex flow, a hollow air core is forme, it is concentric with the nozzle axis. The outflowing thin conical liqui sheet attenuates rapily becoming unstable an isintegrates into ligaments an then rops in the form of a well-efine hollow cone spray [1]. Dombrowski an Hasson [6] also inicate that the motion in the swirl chamber is complex an the mechanisms of flow within the chamber an the resultant spray are not fully unerstoo. Taylor [7] has shown that although the motion in the main stream can be consiere irrotational, viscous effect on the retare bounary layer cannot be neglecte. The liqui in contact with the atomizer walls cannot rotate fast enough to hol it in a circular path against the raial pressure graient balancing the centrifugal motion an consequently a current irecte towars the orifice is set up through the surface layer. Pressure swirl atomizer essentially consists of three main elements inlet tangential ports, swirl chamber an exit orifice. The exit orifice is precee by a swirl chamber with a certain contraction or convergence. The inlet is one or more cylinrical or rectangular channels positione tangentially to the swirl chamber as shown in Fig.1. The swirl chamber contains a strongly swirl motion of the liqui fuel an the central air core. In the chamber, a portion of the swirl energy is converte into axial velocity an the liqui fuel flows out of the nozzle in the form of a hollow cone. The exit orifice serves as the ischarge outlet for the atomizer an contains holes in which the liqui is ischarge. The size of the hole is usually of the orer of a tenth of millimetre or less [8]. The internal flow of pressureswirl atomizers has been stuie numerically [9, 1].The internal flow characteristics in pressure-swirl atomizers are important, because they govern the thickness of the sheet forme at the ischarge orifice. It also governs the magnitue of the axial, tangential an raial velocity components of the sheet an hence the break-up of the film an characteristics of the resulting spray. The most important internal parameters in pressure swirl atomizers are: the pressure exerte by the flui on the walls, the axial an the tangential components of the velocity an the air core characteristics. These characteristics significantly influence the ischarge coefficient at the exit orifice, the spray cone angle an the liqui film thickness. The pressure on the internal walls of the atomizer also influences the swirling motion an the initiation of the central air core. Horvay an Leukel [1] observe that the pressure in swirl atomizer is almost constant in the swirl chamber, a sharp rop in the contraction zone an further ecreases in the exist orifice. The liqui velocity is an essential factor that affects the egree of atomization an primarily epens on the injection pressure. The velocity has three components: the axial, tangential an the longituinal. The formation of a central air core is another most important feature IN: IJET Publications UK. All rights reserve. 145

2 International Journal of Engineering an Technology (IJET) olume 6 No. 5, May, 216 of the flow in a simplex nozzle. The size of the air core etermines the effective flow area at the ischarge orifice an thus controls the coefficient of ischarge, which is one of the important performance parameters of the nozzle [11]. vector of boy forces per unit mass an b ɸ represents sources an sinks of ɸ [18]. To account for the free surface an allow for arbitrary eformation, an aitional equation nees to be solve for the volume fraction α of the liqui phase (C3H8) α + α(v v t b). ns = The liqui propane (C3H8) an (air) are consiere as two immiscible components of a single effective flui, whose properties are assume to vary accoring to the volume fraction of each component as follows for the ensity ρ an viscosity μ: (4) Fig.1. Pressure swirl atomizer schematic layout everal approaches are use in the numerical analysis of multiphase flow in swirl atomizers. There are, among others, interface tracking methos, like: volume of flui (OF) [12, 13], level-set [14, 15] an front-tracking[16]. olume of flui (OF) metho use in this stuy has shown to be more flexible an efficient than other methos when treating complicate free bounary configurations. The OF moel is a simple multi-phase moel that is well suite to simulate flows that consist of two or more immiscible fluis. The moel assumes that all immiscible fluis present in each control volume share the same velocity, pressure an temperature fiels. As a result of this assumption, the same set of basic governing equations escribing momentum, mass an energy transport in a single-phase flow is solve for an equivalent flui whose physical properties are calculate as function of its respective phases volume of fraction [17]. 2. NUMERICAL IMULATION For a viscous three-imensional flow, the flow is assume to be governe by the Reynols-average Navier-tokes equations (RAN), in which the turbulence effects are inclue via eyviscosity moel. In this case, the continuity equation, three momentum components equations, an two equations for turbulence properties are solve. The equations are: Mass conservation ρ + ρv(v v t b). ns = Momentum conservation ρ + ρv(v v t b). ns = (T pi). ns + ρb (2) Generic transport equations for scalar quantities ρɸ + ρɸ(v v t b). ns = Γ ɸ. ns + ρb ɸ (3) where ρ stans for flui ensity, v is the flui velocity vector an v b is the velocity of the C surface, n is the unit vector normal to the C surface with area an volume. T stans for the stress tensor (expresse in terms of velocity graient an ey viscosity), p is the pressure, I is the unit tensor, ɸ stans for the scalar variable, Γ =.7 is the iffusivity coefficient, b is the (1) ρ = ρ 1 c + ρ 2 (1 α) (5) μ = μ 1 c + μ 2 (1 α) (6) Fig.2. Geometry an bounary conitions, three-imensional Computations are performe courtesy of the CFD coe TAR CCM R8 for Winows 64.The equations (1)-(6) are solve using the finite-volume metho in association with the IMPLE algorithm an the econ Orer Upwin scheme. The IMPLE algorithm uses a relationship between velocity an pressure corrections to enforce mass conservation an to obtain pressure fiel. The econ Orer iscretization is more accurate than the First Orer upwin scheme an more suite for practical engineering problems. Two equations stanar k-ε moel has been aopte for the computation of turbulence in the fluis since there is no conclusive information available in the literature concerning accurate an suitable moification of the k-ε moel for multiphase flow. The geometry shown in Fig.2 was create using CAD package (oliworks) an importe into the TAR- CCM+. The moel consists of two parts the nozzle part which is attache to the computation omain just to stuy the flow at the exist orifice of 2mm in iameter. The computation omain imensions are large so that the outlet bounary conitions o not affect the flow. Three-imensional calculations are carrie out on the flow through the velocity inlet2 of 2mm iameter with ensity of propane liqui (ρ l = 8kg/m 3 ) an ensity of air (ρ a = IN: IJET Publications UK. All rights reserve. 146

3 International Journal of Engineering an Technology (IJET) olume 6 No. 5, May, kg/m 3 ) through velocity inlet1 having 2mm of iameter as shown in Fig.3. The walls represent the soli walls of the nozzle an the computation omain. The stanar wall functions are use to moel the near-wall regions with no-slip conitions. Pressure outlet is specifie for the flow out. The inlet bounary conitions use to perform the calculations for the liqui are: 3bars for the injection pressure, 1 for volume fraction, 1% for turbulent intensity,.4 mm for turbulent length scale an 1. m/s for the velocity magnitue. An for the air inlet the value remains the same except for the velocity magnitue which is 1 m/s. The outlet bounary conitions are 1% for turbulent intensity, volume fraction of one for both liqui an air an.1m for the turbulent length scale. A non-uniform mesh gri compose of 5472 tetraheral cells is use for the computation an shown in Fig.4. The istribution of the mesh was one such that the swirl chamber an the cyliner have fine an coarse meshes respectively. The refine gri spacing on the swirl chamber was about.15 mm an a coarse mesh of 1. mm on the computation omain with a growth factor of 2.. This is to reuce the computational time an for the 3.2GHz Intel(R) Xeon(R) processor to accommoate. The total computational time was about two (2) ays. Fig.4. Computational gri, three-imensional. 3. REULT AND DICUION Fig.3. Geometry an bounary conitions, two-imension Fig.5. Contour plot of volume fraction of propane on the crosssectional plane IN: IJET Publications UK. All rights reserve. 147

4 International Journal of Engineering an Technology (IJET) olume 6 No. 5, May, 216 Fig.6. Contour plot of tangential velocity on cross-sectional plane The Fig.5 shows the contour plot of volume fraction of propane on cross-sectional plane.the volume fractions are colour coe such that the re represents the maximum an the blue inicates the minimum volume for the liqui. The volume of fraction is 1 an for the liqui an air phase respectively. It can be observe that the liqui volume fraction is quite symmetrical on the spray axis. Fig.6 shows the tangential velocity istribution on the cross plane through the swirl chamber. Maximum an minimum tangential velocities can be observe iametrically opposite. Fig.8. ortices in the atomizer visualise by path lines On the vertical plane through the pressure swirl atomizer Fig. 7 shows the contour plot of the axial velocity obtaine in the numerical simulation. Regions of higher velocity in the liqui are foun near the wall. The liqui passes the concave wall, giving rise to hyroynamic instabilities which result in the formation of the vortices as visualize by the path lines shown in Fig.8. Fig.7. Axial velocity scene within the atomizer Fig. 9. Contour plot of volume of fraction of air on the crosssectional plane IN: IJET Publications UK. All rights reserve. 148

5 velocity: maggnitue m/s pressure. Pa velocity componet m/s International Journal of Engineering an Technology (IJET) olume 6 No. 5, May, x (mm) Axial elocity (m/s) Tangential velocity Fig.12. Axial an tangential velocity profile in the swirl chamber.6 Fig.1. olume fraction of propane on the vertical plane Fig.1 presents the volume fraction of propane (C3H8) on the vertical plane. The liqui core in the mile (yellow an green) is inuce by the motion of the air shown in Fig.9. The volume is maximum in the vicinity of the inlet2, inicate in the re colour, an ecreases towars the exit orifice. Qualitatively, the iameter of the liqui is slightly larger in the convergent part of the chamber than its size in the swirl chamber an reuces in size at the exit orifice. This is in general agreement with De Keukelare [8] who experimentally iscovere that there is a slight reuction in iameter of flui within the outlet of the convergence an exit orifice, an is attribute to the a vena contracta effect, ue to the liqui having to negotiate a sharp ben. The interface between the propane (C3H8) an air becomes unsteay isplaying waves of small amplitue along its surface. The waves originate at the stagnation point an propagate towars the exit. Their amplitue seems to be increasing with ecreasing axial istance. After exit, violent movement is notice between the liqui propane (C3H8) an the air x (mm) contraction part x (mm) swirl chmaber Fig.13. Pressure profile of the liqui on the internal walls of the atomizer Fig.11. shows the graphs of velocity magnitue for the various positions on a horizontal line rawn across the section plane. It can be observe that the velocity magnitues are high at the centre positions an ecrease quiet linearly towars the swirl chamber walls. The general profile of the velocity magnitues, taken on the probe line across the section plane, is quite similar to the measure velocity profiles at the inlet level by Horvay an Leuckel [1]. The raial istance istribution of the axial an tangential velocities of the liqui in the pressure swirl atomizer is shown in Fig.12. It is observe that the tangential component of the velocity is relatively higher than axial velocity in the swirl chamber although it falls below the axial velocity slightly towars the walls in the swirl chamber. The pressure istributions of the liqui on the internal walls of the atomizer, for the swirl chamber an contraction regions, are compare an presente in Fig.13. It is euce from the graph that the pressure is higher in the contraction region than the swirl chamber. Fig.11. Profile of velocity magnitue on a horizontal line across the swirl chamber IN: IJET Publications UK. All rights reserve. 149

6 presure Pa velocity componetnt m/s velocity: magnitue m/s Raial velocity m/s International Journal of Engineering an Technology (IJET) olume 6 No. 5, May, y (mm) Fig.14. elocity magnitue on symmetry line on the vertical plane 4.E-2 3.E-2 2.E-2 1.E-2.E E-2-2.E-2-3.E-2 y(mm) Axial elocity (m/s) Tangential velocity Fig.15. elocity components on symmetry line on the vertical plane 6.E-1 5.E-1 4.E-1 3.E-1 2.E-1 1.E-1.E E-1 5 y (mm) Fig.16. Pressure profile on symmetry line on the vertical plane Fig. 17. Raial velocity on symmetry line on the vertical plane Fig.14 presents the graph obtaine for the velocity magnitues on the spray axis (symmetry axis). The velocity magnitues are higher in the atomizer an ecrease in the computational omain. The other parameters such as axial an tangential velocities, pressure an raial velocity show similar trens on the symmetry axis on the vertical plane (Figs.15-17). 4. CONCLUION Multiphase moelling in pressure swirl atomizer is one with CFD commercial coe TAR-CCM+ using the volume of flui (OF) approach in moelling the internal characterises of pressure swirl atomizer. cale scene of volume fraction of the liqui propane an air as well as vorticity have been shown. Axial an tangential velocities resulting from the flow have been presente an analyse. The general profiles of the results are similar to the trens in the literature. REFERENCE y (mm) Lefebvre, A., Atomization an sprays. ol : CRC press. Lefebvre, A.H. an D. Hallal, Gas Turbine Alternative Fuels an Emissions. CRC, Boca Raton, FL, 21. Lefebvre, A.H., Gas turbine combustion. 1998: CRC press. Khavkin, Y.I., Theory an practice of swirl atomizers. 23: CRC Press. Bayvel, L. an Z. Orzechowski, Liqui Atomization, Combustion: An International eries. 1993, Taylor & Francis. Dombrowski, N. an D. Hasson, The flow characteristics of swirl (centrifugal) spray pressure nozzles with low viscosity liquis. AIChE Journal, (4): p Taylor, G., The bounary layer in the converging nozzle of a swirl atomizer. The Quarterly Journal of Mechanics an Applie Mathematics, (2): p De Keukelaere, H., The Internal Flow in a wirl Atomizer Nozzle IN: IJET Publications UK. All rights reserve. 15

7 International Journal of Engineering an Technology (IJET) olume 6 No. 5, May, 216 Yule, A. an J. Chinn, The internal flow an exit conitions of pressure swirl atomizers. Atomization an prays, 2. 1(2): p Horvay, M. an W. Leuckel, Experimental an theoretical investigation of swirl nozzles for pressure-jet atomization. German chemical engineering, (5): p Datta, A. an. om, Numerical preiction of air core iameter, coefficient of ischarge an spray cone angle of a swirl spray pressure nozzle. International journal of heat an flui flow, 2. 21(4): p Hirt, C.W. an B.D. Nichols, olume of flui (OF) metho for the ynamics of free bounaries. Journal of computational physics, (1): p Ménar, T.,. Tanguy, an A. Berlemont, Coupling level set/of/ghost flui methos: aliation an application to 3D simulation of the primary break-up of a liqui jet. International Journal of Multiphase Flow, (5): p ethian, J.A., Level set methos an fast marching methos: evolving interfaces in computational geometry, flui mechanics, computer vision, an materials science. ol : Cambrige university press. Osher,., R. Fekiw, an K. Piechor, Level set methos an ynamic implicit surfaces. Applie Mechanics Reviews, : p. 15. Tryggvason, G., et al., A front-tracking metho for the computations of multiphase flow. Journal of Computational Physics, (2): p Johannessen,.R., Use of CFD to tuy Hyroynamic Loas on Free-Fall Lifeboats in the Impact Phase.: A verification an valiation stuy Guie, U., tar-ccm+ ersion 8.4. CD-aapco-213, 213. IN: IJET Publications UK. All rights reserve. 151